1. Field of the Invention
The present invention relates to a method of and an apparatus for measuring oxygen concentration in a sample gas with a limiting current oxygen sensor, and more particularly, it relates to measuring method and apparatus which are suitable for measuring oxygen concentration in a sample gas, such as an exhaust gas, for example, containing a great deal of an interference gas.
2. Description of the Background Art
A limiting current oxygen concentration measuring apparatus is provided with a limiting current oxygen sensor comprising electrodes which are formed on both sides of a stabilized zirconia member. When a voltage is applied across the cathode and the anode of the oxygen sensor during bringing a sample gas into contact with the cathode, a current flows through carriers of oxygen ions contained in the sample gas by pumping action of the stabilized zirconia member. Thus, oxygen contained in the sample gas is discharged toward the anode through the stabilized zirconia member. At this time, the current becomes constant with respect to the oxygen concentration of the sample gas in a certain range of the voltage across the electrodes. Such a constant current is called a limiting current. Since the oxygen concentration has linear relation to the limiting current, it is possible to detect the oxygen concentration by measuring the value of the limiting current.
A limiting current oxygen sensor employing such a principle is generally adapted to measure oxygen concentration in a clean gas, or a gas which is approximate in composition to the atmospheric gas. When the sample gas is an exhaust gas from an automobile or a factory and contains CO.sub.2 in high concentration, however, CO.sub.2 serves as an interference gas for oxygen in the oxygen sensor to hinder correct measurement of oxygen concentration. In general, therefore, the oxygen sensor is not employed for measurement of a gas such as an exhaust gas containing CO.sub.2 in high concentration.
When CO.sub.2 contained in a sample gas having high CO.sub.2 concentration is absorbed by an absorbent in order to measure oxygen concentration with the oxygen sensor, the composition of the sample gas is so changed that oxygen concentration cannot be correctly attained in a sampling point.
In the aforementioned oxygen sensor, further, it is necessary to limit the inflow of oxygen molecules by some means. When the inflow is limited by a small hole, for example, no problem is caused in measurement of a gas which is substantially unchanged in spun gas composition. If a gas containing a great deal of foreign gas molecules, such as those of CO.sub.2 contained in an exhaust gas, in percentage order, however, physical properties of the sample gas are so changed that an indicated value of oxygen concentration cannot be correctly attained.